Infrared spectroscopy of pyrrole-2-carboxaldehyde and its dimer: a planar beta-sheet peptide model?

Intermolecular interactions relevant for antiparallel beta-sheet formation between peptide strands are studied by Fourier transform infrared spectroscopy of the low temperature, vacuum-isolated model compound pyrrole-2-carboxaldehyde and its dimer in the N-H and C=O stretching range. Comparison to quantum chemical predictions shows that even for some triple-zeta quality basis sets, hybrid density functionals and Møller-Plesset perturbation calculations fail to provide a consistent and fully satisfactory description of hydrogen bond induced frequency shifts and intensity ratios in the double-harmonic approximation. The latter approach even shows problems in reproducing the planar structure of the dimer and the correct sign of the C=O stretching shift for standard basis sets. The effect of matrix isolation is modeled by condensing layers of Ar atoms on the isolated monomer and dimer. The dimer structure is discussed in the context of the peptide beta-sheet motif.